1. Field of the Invention
The present invention relates to a master information carrier for magnetic transfer, which bears signal patterns on its surface, so as to magnetically transfer desired magnetized patterns onto a magnetic recording medium. The present invention also relates to a magnetic transfer method using the master information carrier. The present invention also relates to a magnetic recording medium.
2. Description of the Related Art
Conventionally, a magnetic transfer method for transferring magnetized patterns corresponding to information, carried by a master information carrier, onto a magnetic recording medium (slave medium) is well known. In this method, the master information carrier for magnetic transfer, which carries the information to be transferred, represented by very fine uneven patterns, is placed in close contact with the slave medium. Then, a magnetic field for transfer is applied to the master information carrier and the slave medium, which are in close contact with each other, and magnetized patterns are transferred onto the slave medium. In the magnetic transfer method, recording may be performed statically without changing the relative position of the master information carrier and the slave medium. Further, the magnetic transfer method has the advantage that the information can be recorded on the slave medium in a very short time (for example, please refer to Japanese Unexamined Patent Publication Nos. 10(1998)-040544, and 10(1998)-269566, or the like).
The application of the magnetic transfer technique to recording of servo signals for positioning a magnetic head on a magnetic recording medium is already proposed in Japanese Unexamined Patent Publication Nos. 10(1998)-040544 and 10(1998)-269566, or the like.
However, there is a technical problem in the magnetic transfer method. When signals are transferred from the master information carrier onto the slave medium, an unclear magnetic recording area (reversely magnetized area) is produced on the slave medium. If the unclear magnetic recording area is present on the slave medium, when the magnetized patterns are read out, a sub-pulse is detected in the waveform of the magnetized patterns. Therefore, there is a possibility that the sub-pulse is recognized as a reproduction signal by a magnetic reproduction apparatus, which may cause an error in reproduction. A method for observing the sub-pulse, generated by the unclear magnetic recording area, is disclosed in U.S. patent application Publication No. 20020044368, and Japanese Unexamined Patent Publication No. 2002-042301, for example.
However, the studies by the inventors of the present invention have revealed that it is possible to suppress generation of the sub-pulse considerably. Generation of the sub-pulse may be suppressed if a substrate made of a magnetic material is used as the substrate of the master information carrier, or if transfer conditions such as a pressure, applied to the master information carrier and the slave medium to place them in close contact with each other, is adjusted.
However, when servo signals are transferred, for example, an unclear magnetic recording area is produced on a slave medium after magnetic transfer. The unclear magnetic recording area is produced in a data area when the data area and a servo area are adjacent to each other in the direction along a track. Therefore, it is difficult to completely eliminate a sub-pulse, generated by the unclear magnetic recording area. An example will be described with reference to schematic diagrams illustrated in FIGS. 8A to 8C.
FIG. 8A is a schematic diagram illustrating a sectional view of a part of a master information carrier in the direction along a track. FIG. 8B is a schematic diagram illustrating magnetized patterns in a corresponding part of a slave medium, onto which the magnetized patterns are transferred from the master information carrier, illustrated in FIG. 8A. FIG. 8C is a schematic diagram illustrating a readout waveform, which is detected from the magnetized patterns. As illustrated in FIG. 8A, uneven patterns corresponding to desired information are formed on a master information carrier 101. The uneven patterns include protrusions and depressions, which have various bit lengths. If the minimum bit length (length in the direction along a track) among the various bit lengths is Bmin, there are many depressions, each having a length L, which satisfies L≧4Bmin. The master information carrier 101 includes a substrate 101a, made of a magnetic material such as Ni, for example. The substrate 101a bears uneven patterns corresponding to the desired information on its surface. The master information carrier 101 also includes a magnetic layer 101b, which is provided on the surface of the uneven patterns of the substrate 101a. The surface of the master information carrier 101, which bears the magnetic layer 101b, and a magnetic recording layer of the slave medium, on which direct current magnetization has been carried out in advance in one direction along the track, are placed in close contact with each other. While they are in close contact with each other, a magnetic field for transfer is applied in a direction opposite to that of the direct current magnetization. Accordingly, the magnetized patterns as illustrated in FIG. 8B are recorded on the slave medium. At this time, unclear magnetic recording areas 104 are produced at both ends of an area corresponding to the depression, which has the length L. Further, the readout waveform as illustrated in FIG. 8C is detected from the magnetized patterns, which are illustrated in FIG. 8B. As illustrated in FIG. 8C, sub-pulses 105, which correspond to the unclear magnetic recording areas 104, are detected. Specifically, the sub-pulses are detected at a reproduction apparatus for reproducing data from magnetic recording mediums. Therefore, there is a possibility that the detected sub-pulses may be incorrectly recognized as reproduction signals. For example, when sub-pulses are generated at both ends of a servo area, the timing of the beginning and end of the servo signal area is incorrectly recognized. Therefore, the servo timing is shifted, and there is a possibility that an error such as failure in servo tracking will be caused.